1. Field of the Invention
The invention relates generally to fluorescent probes which include a fluorescent reporter molecule and a fluorescent quencher molecule. More specifically, the invention relates to fluorescent probes which include a fluorescent reporter molecule and a fluorescent quencher molecule which may be used in hybridization assays and in nucleic acid amplification reactions, especially polymerase chain reactions (PCR).
2. Description of Related Art
Fluorescent reporter molecule--quencher molecule pairs have been incorporated onto oligonucleotide probes in order to monitor biological events based on the fluorescent reporter molecule and quencher molecule being separated or brought within a minimum quenching distance of each other. For example, probes have been developed where the intensity of the reporter molecule fluorescence increases due to the separation of the reporter molecule from the quencher molecule. Probes have also been developed which lose their fluorescence because the quencher molecule is brought into proximity with the reporter molecule. These reporter--quencher molecule pair probes have been used to monitor hybridization assays and nucleic acid amplification reactions, especially polymerase chain reactions (PCR), by monitoring either the appearance or disappearance of the fluorescence signal generated by the reporter molecule.
As used herein, a reporter molecule is a molecule capable of generating a fluorescence signal. A quencher molecule is a molecule capable of absorbing the fluorescence energy of an excited reporter molecule, thereby quenching the fluorescence signal that would otherwise be released from the excited reporter molecule. In order for a quencher molecule to quench an excited fluorophore, the quencher molecule must be within a minimum quenching distance of the excited reporter molecule at some time prior to the reporter molecule releasing the stored fluorescence energy.
Probes containing a reporter molecule--quencher molecule pair have been developed for hybridization assays where the probe forms a hairpin structure, i.e., where the probe hybridizes to itself to form a loop such that the quencher molecule is brought into proximity with the reporter molecule in the absence of a complementary nucleic acid sequence to prevent the formation of the hairpin structure. WO 90/03446; European Patent Application No. 0 601 889 A2. When a complementary target sequence is present, hybridization of the probe to the complementary target sequence disrupts the hairpin structure and causes the probe to adopt a conformation where the quencher molecule is no longer close enough to the reporter molecule to quench the reporter molecule. As a result, the probes provide an increased fluorescent signal when hybridized to a target sequence than when unhybridized. Probes including a hairpin structure have the disadvantage that they can be difficult to design and may interfere with the hybridization of the probe to the target sequence.
Assays have also been developed for identifying the presence of a hairpin structure using two separate probes, one containing a reporter molecule and the other a quencher molecule. Mergney, et al., Nucleic Acids Research, 22:6 920-928 (1994). In these assays, the fluorescence signal of the reporter molecule decreases when hybridized to the target sequence due to the quencher molecule being brought into proximity with the reporter molecule.
One particularly important application for probes including a reporter--quencher molecule pair is their use in nucleic acid amplification reactions, such as polymerase chain reactions (PCR), to detect the presence and amplification of a target nucleic acid sequence. In general, nucleic acid amplification techniques have opened broad new approaches to genetic testing and DNA analysis. Arnheim and Erlich, Ann. Rev. Biochem., 61: 131-156 (1992). PCR, in particular, has become a research tool of major importance with applications in, for example, cloning, analysis of genetic expression, DNA sequencing, genetic mapping and drug discovery. Arnheim and Erlich, Ann. Rev. Biochem., 61: 131-156 (1992); Gilliland et al., Proc. Natl. Acad. Sci., 87: 2725-2729 (1990); Bevan et al., PCR Methods and Applications, 1: 222-228 (1992); Green et al., PCR Methods and Applications, 1: 77-90 (1991); Blackwell et al., Science, 250: 1104-1110 (1990).
The widespread applications of nucleic acid amplification techniques has driven the development of instrumentation for carrying out the amplification reactions under a variety of circumstances. Important design goals for such instrument development have included fine temperature control, minimization of sample-to-sample variability in multi-sample thermal cycling, automation of pre- and post-reaction processing steps, high speed temperature cycling, minimization of sample volumes, real time measurement of amplification products and minimization of cross contamination, for example, due to "sample carryover". In particular, the design of instruments permitting amplification to be carried out in closed reaction chambers and monitored in real time would be highly desirable for preventing cross-contamination. Higuchi et al., Biotechnology, 10: 413-417 (1992) and 11: 1026-1030 (1993); and Holland et al., Proc. Natl. Acad. Sci., 88: 7276-7280 (1991). Clearly, the successful realization of such a design goal would be especially desirable in the analysis of diagnostic samples, where a high frequency of false positives and false negatives, for example caused by "sample carryover", would severely reduce the value of an amplification procedure. Moreover, real time monitoring of an amplification reaction permits far more accurate quantification of starting target DNA concentrations in multiple-target amplifications, as the relative values of close concentrations can be resolved by taking into account the history of the relative concentration values during the reaction. Real time monitoring also permits the efficiency of the amplification reaction to be evaluated, which can indicate whether reaction inhibitors are present in a sample.
Holland et al. (cited above), U.S. Pat. No. 5,210,015 to Gelfand, et al. and others have proposed fluorescence-based approaches to provide real time measurements of amplification products during PCR. Such approaches have either employed intercalating dyes (such as ethidium bromide) to indicate the amount of double-stranded DNA present, or they have employed probes containing fluorescence-quencher pairs (also referred to as the "Taq-Man" approach) where the probe is cleaved during amplification to release a fluorescent molecule whose concentration is proportional to the amount of double-stranded DNA present. During amplification, the probe is digested by the nuclease activity of a polymerase when hybridized to the target sequence to cause the fluorescent molecule to be separated from the quencher molecule, thereby causing fluorescence from the reporter molecule to appear.
The Taq-Man approach, illustrated in FIG. 1, uses an oligonucleotide probe containing a reporter molecule--quencher molecule pair that specifically anneals to a region of a target polynucleotide "downstream", i.e. in the direction of extension of primer binding sites. The reporter molecule and quencher molecule are positioned on the probe sufficiently close to each other such that whenever the reporter molecule is excited, the energy of the excited state nonradiatively transfers to the quencher molecule where it either dissipates nonradiatively or is emitted at a different emission frequency than that of the reporter molecule. During strand extension by a DNA polymerase, the probe anneals to the template where it is digested by the 5'.fwdarw.3' exonuclease activity of the polymerase. As a result of the probe being digested, the reporter molecule is effectively separated from the quencher molecule such that the quencher molecule is no longer close enough to the reporter molecule to quench the reporter molecule's fluorescence. Thus, as more and more probes are digested during amplification, the number of reporter molecules in solution increases, thus resulting in an increasing number of unquenched reporter molecules which produce a stronger and stronger fluorescent signal.
Three main factors influence the utility of reporter-quencher molecule pair probes in hybridization and amplification assays. The first factor is the effectiveness of the quencher molecule on the probe to quench the reporter molecule. This first factor, herein designated "RQ.sup.- ", can be characterized by the ratio of the fluorescent emissions of the reporter molecule to the quencher molecule when the probe is not hybridized to a complementary polynucleotide. That is, RQ.sup.- is the ratio of the fluorescent emissions of the reporter molecule to the fluorescence of the quencher molecule when the oligonucleotide probe is in a singlestranded state. Influences on the value of RQ.sup.- include, for example, the particular reporter and quencher molecules used, the spacing between the reporter and quencher molecules, nucleotide sequence-specific effects, and the degree of flexibility of structures, e.g., linkers, to which the reporter and quencher molecules are attached, and the presence of impurities. Wo et al., Anal. Biochem., 218: 1-13 (1994); and Clegg, Meth. Enzymol., 211: 353-388 (1992). A related quantity RQ.sup.+, refers to the ratio of fluorescent emissions of the reporter molecule to the quencher molecule when the oligonucleotide probe is hybridized to a complementary polynucleotide.
A second factor is the efficiency of the probe to hybridize to a complementary polynucleotide. This second factor depends on the probe's melting temperature, T.sub.m, the presence of a secondary structure in the probe or target polynucleotide, the annealing temperature, and other reaction conditions.
A third factor is the efficiency with which the DNA polymerase 5'.fwdarw.3' exonuclease activity cleaves the bound probe between the reporter molecule and quencher molecule. This efficiency depends on such factors as the proximity of the reporter or quencher to the 5' end of the probe, the "bulkiness" of the reporter or quencher, and the degree of complementarity between the probe and target polynucleotide. Lee et al., Nucleic Acids Research, 21: 3761-3766 (1993).
Since quenching depends on the physical proximity of the reporter molecule to the quencher molecule, it was previously assumed that the quencher and reporter molecules must be attached to the probe such that the quencher molecule remains at all times within the maximum distance at which the quencher molecule can quench the reporter molecule, this distance generally being a separation of about 6-16 nucleotides. Lee et al. Nucleic Acids Research, 21: 3761-3766 (1993); Mergny et al., Nucleic Acids Research 22: 920-928 (1994); Cardullo et al., Proc. Natl. Acad. Sci., 85: 8790-8794 (1988); Clegg et al., Proc. Natl. Acad. Sci., 90: 2994-2998 (1993); and Ozaki etal., Nucleic Acids Research, 20: 5205-5214 (1992). This short separation between the reporter molecule and the quencher molecule is typically achieved by attaching one member of the reporter-quencher pair to the 3' or 5' end of the probe and the other member to an internal base 6-16 nucleotides away.
There are at least two significant disadvantages associated with attaching a reporter or quencher molecule to an internal base. Attaching a reporter or quencher molecule to an internal nucleotide typically involves more difficult chemistry than the chemistry required to attach the molecule to a terminal nucleotide. In addition, attachment of a reporter or quencher molecule to an internal nucleotide can adversely affect the hybridization efficiency of the probe. Ward et al., U.S. Pat. No. 5,328,824; and Ozaki et al. Nucleic Acids Research, 20: 5205-5214 (1992).
A need currently exists for effective oligonucleotide probes containing a fluorescent reporter molecule and a quencher molecule for use in hybridization and nucleic acid amplification assays. Accordingly, a need exists for probes which exhibit distinguishable fluorescence characteristics when hybridized and not hybridized to a target nucleic acid sequence. A further need exists for probes where the reporter molecule and quencher molecule are positioned on the probe such that the quencher molecule can effectively quench the fluorescence of the reporter molecule. A further need exists for probes which are efficiently synthesized. Yet a further need exists for the reporter molecule and quencher molecule to be positionable on the probe such that the reporter and quencher molecules do not adversely impact the hybridization efficiency of probe. These and further objectives are provided by the probes and methods of the present invention.